The present invention relates to a device for eliminating or cancelling the ghosts.
There has been deviced and demonstrated a method called "delay line cancellation" for cancelling the ghost in television receivers. According to this method, a ghost cancellation circuit consisting of a variable delay line, a circuit for controlling the gain and phase and an adder is inserted between a video intermediate-frequency amplifier stage and a detector stage. The output from the video intermediate-frequency amplifier stage is delayed through the variable delay line and the circuit for controlling the gain and phase of the output from the delay line, which make up a delay channel and is added to the output signal transmitted from the video intermediate-frequency amplifier stage to the adder through an undelayed channel, whereby the ghost signals are cancelled.
Another ghost cancellation circuit based on the delay line cancellation method is substantially similar in construction to that described above except that this circuit is inserted into a video signal stage succeeding the detector stage.
In a further example, two variable delay lines are inserted into delay channels before and after the detector stage, and the delayed and undelayed signals are added in the video signal stage.
The delay line cancellation method described above is based upon the theory that the desired signal which has been delayed in time with respect to the original desired signal and whose intensity or amplitude is varied with respect to the original desired signal is superposed upon the original desired signal so that the ghost is produced and the concept developed from this theory that the ghost signal may be cancelled in a television receiver if the desired signal is delayed by a time equal to the delay of the ghost signal, the amplitude or intensity of the desired signal thus delayed is made equal to that of the ghost signal and the thus delayed and amplitude or intensity controlled signal is subtracted from the desired signal. To design and construct a ghost cancellation circuit based upon the above theory and concept is sometimes easy and sometimes difficult depending upon what kind or type of a variable delay line is employed.
The conventional variable delay lines may be in general divided into the following three types:
(A) accoustic surface-wave delay lines; PA1 (B) LC distribution or lumped parameter delay lines; and PA1 (C) semiconductor delay lines such as CCD and BBD.
For instance, an accoustic surface-wave delay line of type (A) consists of a substrate made of a single crystal of lithium noibate and straight input and output electrodes disposed on the surface of the substrate at right angles so that the time delay caused by the propagation of the signal by the surface-wave may be utilized. Therefore this type of delay line is only used in a IVF stage in a television receiver. Furthermore, the time delays are discrete; that is, the delay cannot be continuously varied because it is impossible to arrange input and output electrodes infinitely. As a result, the first mentioned ghost cancellation circuit is very unsatisfactory in practice.
In the second and third mentioned ghost cancellation circuits, the video signal is delayed so that the continuous delay may be possible. However, when the video signal is delayed too long, the frequency characteristic is adversely affected so that it becomes impossible to transmit better images. Therefore the second mentioned circuit is also unsatisfactory in practice.
In the third mentioned circuit, the output from the video intermediate-frequency amplifier stage is delayed by a substantial degree in the first delay line preceding to the detector stage, and then delayed again by a very small degree in the second delay line after the detector stage. Therefore this circuit is regarded most satisfactory at present. In the first mentioned circuit, the desired signal and the ghost signal are added before they pass through the detector stage so that the phase relation between the desired and ghost signals may be negligible. However, in the third mentioned circuit, the signal transmitted through the delay channel and the undelayed signal are added after the detector stage so that the high-frequency phase relation between them cannot be held negligible. If this relation is neglected, the effective ghost cancellation or elimination is impossible.